1. Field of the Invention
The present invention relates to a ball screw apparatus used in various industrial machines.
2. Background Art
In a feeding mechanism or the like of various industrial machines, as an apparatus of converting rotational movement into linear movement, a ball screw of a tube circulating type is used.
FIG. 17 shows an example of a circulating tube type ball screw apparatus of a prior art, and according to the ball screw apparatus 1, a screw shaft 3 having a screw groove 2 in a spiral shape at an outer peripheral face thereof is screwed with a nut 6 having a screw groove 4 in a spiral shape in correspondence with the screw groove 2 at an inner peripheral face thereof.
The screw groove 4 of the nut 6 and the screw groove 2 of the screw shaft 3 are opposed to each other to form a load track in a spiral shape between the both members, and a number of balls 5 as rolling members are rollably charged to the load track. Further, by rotating the screw shaft 3 (or nut 6), the nut 6 (or screw shaft 3) is made to move in an axial direction via rolling of the balls 5.
Further, a portion of an outer peripheral face of the nut 6 is constituted by a flat face, the flat face is formed with a set of 2 pieces holes 7 communicated with the both screw grooves 2, 4 to ride over the screw shaft 3, and by fitting both ends of a circulating tube 8 substantially in a channel-like shape as an example of a ball circulating member to the set of holes 7, the balls 5 revolving along the load track between the both screw grooves 2, 4 are scooped up by the circulating tube 8 from a middle of the load track to return to the original track load, thereby, the balls 5 are infinitely circulated.
Meanwhile, according to the ball screw apparatus, as preload types, generally, there are an over size preload type (P) for applying preload by inserting a ball (steel ball) having a size larger than the screw groove by several μm, and a double nut preload type (D) and an offset lead preload type (Z) for applying preload by constituting the screw groove by a number of rows and slightly shifting leads of respective rows opposed to each other.
According to the double nut preload type (D), the nut is divided in two in an axial direction and therefore, cost is required for a fastening part and time is taken for operation of integrating the fastening part and therefore, particularly in the case of small lead, the offset lead preload type changing the lead at the middle of a single nut is frequently used.
As circulating types of the ball screw apparatus, other than the above-described circulating tube type, there are types referred to as a frame type and an end cap type and the types are generally provided with characteristics as shown by Table 1.
TABLE 1Tube typeFrame typeEnd cap typeNumber of turnsA number ofOnly one turnA number ofper rowturns possiblepossibleturnsimpossibleMulti-circuitPossiblePossibleImpossibleformationPreload typeD, Z, PD, Z, PP (Z, D)Adaptability ofSmall-middleAdaptable toAdaptable toleadleadsmall leadlarge lead
Although high speed formation or high rated load formation is requested for a ball screw apparatus and therefore, it is necessary to increase a number of balls bearing a load, on the other hand, when an excessively large number of balls are put into one row of a circuit, the circulating types of the frame type and the end cap type which are disadvantageous in view of operability in which balls are rubbed by each other, it is difficult to make high rated load formation and operational property or the like compatible with each other in view of the respective characteristics shown in Table 1.
Meanwhile, the circulating tube type is an external circulating type capable of constituting multi-row formation by completely separating the ball from the screw groove of the nut from the direction of a side face of the nut, the circulating tube type is a circulating type preferable for high rated load formation of particularly, a small lead product.
Further, with high rotation formation of a ball screw apparatus, a speed of impacting the ball to the circulating tube is accelerated. When the impact speed is accelerated, and impact energy is increased, the circulating tube or the screw groove (including both shoulder portions of the screw groove) is destructed to hamper high speed formation and therefore, there is proposed a ball screw apparatus in which a direction of scooping up the ball by the circulating tube is inclined substantially in a tangential direction of the screw shaft and substantially in a direction of a lead angle.
However, when the ball is intended to scoop up in the tangential direction of the screw shaft and in the direction of the lead angle of the screw shaft, the circulating tube is constituted by a complicated shape (Z type instead of simple channel-like type), and when holes opened at the flat face of the nut are simple spot facing holes, the both end portions of the circulating tube cannot be fitted thereto.
Hence, in the prior art, as shown by FIG. 18, there is adopted a method of subjecting the hole 7 formed that the flat face of the nut 6 to spot facing to a considerable degree or working the hole 7 skewedly (in tangential direction) (refer to, for example, JP-UM-A-63-132156).
Incidentally, in a general ball screw, a circulating tube made of a metal is used as a member of circulating a ball, the ball is scooped up to outside of a nut at a tongue portion provided at an end portion of the circulating tube and therefore, there is a difficulty of bringing about vibration, noise or the like when the ball is impacted to the tongue portion of the circulating tube.
Hence, there is proposed a ball screw of a type (hereinafter, referred to as “tangential scooping up type”) using a ball circulating member made of a resin as a member for circulating a ball and scooping up the ball rolling between ball screw grooves of a screw shaft and a nut in a direction of a tangential line brought into contact with a center track circle of a ball by a tongue portion of the ball circulating member to circulate at outside of the nut (refer to JP-A-2003-232421).